Diamonds are a key component utilized in luxury goods, in particular in articles of jewelry, and can have a very great value. The value of a diamond depends on several physical properties of the diamond.
There are four globally accepted standards utilized to assess the quality of a diamond, typically known as the 4C's, which are Clarity, Colour, Cut and Carat Weight. For a diamond, with the exception colour of a diamond which may have a particular or fancy colour, the value of a diamond is highly dependent on what is known as its colourlessness. The more colourless the diamond, the higher.
By way of example, the Gemological Institute of America (GIA) has a colour grade from D to Z, for which the D grade denotes a diamond which is completely colourless, and ranging to a Z grade which denotes a diamond having a significant amount of unwanted colour.
Shown below is the Gemological Institute of America (GIA) colour scale, against which a colour grading is applied, with the grades shown from colourless to light.
GIA COLOR SCALEDEFGHIJKLMNOPQRSTUVWXYZCOLORLESSNEAR COLORLESSPAINTVERY LIGHTLIGHT
Although the human visual recognition of a different diamond colour is not particularly sensitive in particular in relation to diamonds of similar grades, only a slightly change in colour can significantly affect the value of the diamond.
Several factors contribute to the colour of a diamond, the most common and important factor being impurities within a diamond. During the formation process of diamonds, impurities can be easily incorporated. Nitrogen is the most common impurity found in natural diamonds, which produces an unwanted yellow colour. The higher nitrogen content in a diamond, the deeper colour and hence lower colour grade the stone is. Boron can also affect diamond colour of a diamond, but is less common. Diamonds with boron impurity shows light blue colour. There are other impurities also affect diamond colour but they are rare.
Apart from impurities, vacancy defects within a diamond also contribute to colour of a diamond. There are different forms of vacancies, such as isolated vacancy, multivacancy complex, and vacancy combining with impurities, etc.
In some diamonds, due to the ambient pressure conditions during the formation process deep in the earth, the carbon atoms may not form ideal tetrahedral structures, and the tetrahedral structures may be deformed. Such crystal deformation remains in natural diamond can also cause colour changes. For the assessment on the colour of a diamond, the most accepted industry standard and practice to determine a diamond's colour is by trained human eyes.
Using GIA as an example, colour grading personnel are trained for several months utilising standard master stones from a master stone set with assorted colour grades. Moreover, during the colour grading process, a diamond under assessment is compared with the master stones side by side in a controlled environment.
The controlled environment is a standard light box with a white tile to place behind the master stones and testing diamond as a backdrop. Under this standardized environment, the colour of a diamond can be graded by referring it to the master stone with the nearest colour.
A diamond is typically viewed from below at about 45 degrees to the pavilion, with a colour grader looking primarily at the pavilion of the diamond and in a direction towards the table of the diamond.
Repetitive training of colour graders is applied, with a view so that different graders can reproduce the same assessment results, with a view to providing uniformity and consistency between colour grading personnel. Although such a colour grading process is extensively used and under this strict colour grading procedures, the reliability and repeatability of the colour grading methodology are still prone to inconsistencies, and such inconsistencies can cause incorrect grading which can adversely impact upon the value of a diamond.